FindTIFF: Restore undocumented TIFF_LIBRARY variable
Since !8803 (merged) we populate
documented result variables directly from the upstream package.
Set the undocumented legacy TIFF_LIBRARY
variable in this code path
to restore compatibility with projects that were relying on it.
Fixes: #26038
Backport: release
Manual-backport: release-3.29
Manual-backport: release-3.28